This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present disclosure. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present disclosure. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.
Field of the Invention
The present invention relates to the field of fluid separation in connection with natural gas processing. The present invention further relates to a gas-to-liquids conversion process wherein gaseous alkanes are converted into higher molecular weight hydrocarbons using an electron beam reactor to form a high-octane transportation fuel.
General Discussion of Technology
Much of the world's energy is derived from fossil fuels. Fossil fuels include hydrocarbon fluids produced from subsurface reservoirs. Such fuels are generally referred to as “oil and gas.” Fossil fuels also include solid hydrocarbons such as coal, bitumen, lignite, tar and kerogen, which are mined. Solid fossil fuels may be heated for steam generation or electricity generation, or may be pyrolyzed, either in situ or at the surface, to create combustible hydrocarbon fluids.
Hydrocarbon fluids are primarily alkanes, which are molecules consisting only of hydrogen and carbon atoms, with all bonds being single bonds. Alkanes are referred to as “saturated” hydrocarbons as they are saturated with hydrogen. The general formula for saturated hydrocarbons is CnH2n+2 (assuming non-cyclic structures). Saturated hydrocarbons are found as either linear or branched species.
Hydrocarbon fluids are frequently used as petroleum fuel. Most commonly, hydrocarbon fluids are taken through a refining process to form gasoline, diesel, or jet fuel (or naptha). These fuels are generally used as transportation fuels, though lighter hydrocarbon fluids are also frequently used for gas-fired steam or electrical generation. Hydrocarbon gases are also commonly used as heating fuel.
The most common transportation fuel for automobiles in the United States is gasoline. Gasoline is a transparent, petroleum-derived liquid that is used primarily as a fuel in internal combustion engines. It consists mostly of organic compounds obtained by the fractional distillation of petroleum, enhanced with a variety of additives. A 42-gallon barrel of crude oil yields about 19 gallons of gasoline when processed in an oil refinery.
The quality of gasoline as a fuel in internal combustion engines is measured by its octane rating. Gasoline is produced in several grades of octane rating. To acquire the higher octane rating, gasoline is generally derived from “heavier” alkane components such as hexane, heptane, octane, pentane, nonane and decane (C6-C10). These components are in liquid form at generally ambient conditions.
In order to refine crude oil into gasoline, the lighter alkane components such as methane and ethane are flashed off. Indeed, in many hydrocarbon production operations the lighter components are never gathered or sold, but are simply burned or “flared” at the surface using a “gas flare.”
In essence, a vertical gas flare is a gas combustion device. Gas flares are used in the oil and gas industry at petroleum refineries, natural gas processing plants, offshore oil and gas rigs, and at oil and gas production sites. Partly because of the shale gas production boom in the United States, the price of natural gas is suppressed and remains much lower than that of the liquid crude oil. Accordingly, some production companies do not find it economical to transport natural gas to a gathering facility or to a market. Therefore, much natural gas is simply flared.
As a way to avoid the flaring of natural gas and to transport natural gas to market, some producers have liquefied the product. Various methods have been disclosed for liquefying a natural gas stream to assist in transportation. A common approach is to chill the gas stream into a condensable product. For methane and ethane, this generally reduces the volume by about a 600:1 ratio. However, the energy and capital costs required to achieve liquefaction and then transport natural gas liquids (such as by LNG tanker, LNG truck, or by chilled pipeline) can be high.
Another approach is to convert methane and/or ethane into higher alkane mixtures using a metathesis process. U.S. Pat. No. 5,414,176 discloses a process for converting methane to higher hydrocarbons, in particular to C2 to C7 hydrocarbons. The process successively comprises bringing a gas stream consisting essentially of methane into contact with a catalyst comprising a transition metal dispersed over a support based on refractory oxide, then bringing the catalyst into contact with a stream of hydrogen so as to form a gas mixture of higher hydrocarbons and of hydrogen. The process includes subsequently recovering the gas mixture, and separating the higher hydrocarbons from the hydrogen. It is noted that the process comprises two successive contacting operations of the catalyst.
U.S. Pat. No. 7,473,814 also discloses a process for converting methane into ethane by contacting methane with an alkane metathesis metal catalyst. The metal catalyst is selected from metal hydrides, metal organic compounds and mixtures thereof. In one aspect, the metal is chosen from the lanthanides, the actinides and the metals from Groups 2 to 12 of the Periodic Table of the Elements, so as to produce ethane in a proportion of at least 65% by weight with respect to carbon-containing products formed in the process. The process is said to be a single-step process carried out under conditions involving a non-oxidative catalytic coupling of methane. The process is proposed to provide a degree of selectivity by weight for ethane with respect to carbon-containing products formed.
Additional techniques for upgrading gaseous components, namely ethane, by contacting ethane in a gas stream with a metal catalyst have been disclosed. An illustrative patent documents include U.S. Patent Publ. No. 2011/0071331 entitled “Process for Converting Ethane Into Liquid Alkane Mixtures,” where, ethane is upgraded to alkanes having liquid mixtures of 4 carbon atoms or more. See also International Patent Appl. Nos. WO 2006/060692, WO 2003/066552 and WO 2001/04077 wherein chemical reactions involving a metal catalyst are used to upgrade methane into ethane or higher alkane mixtures.
Another solution is to convert the natural gas into hydrocarbon liquids using chemical processing. Fischer-Tropsch technology converts natural gas into “syngas,” which is a mixture of carbon monoxide and hydrogen, followed by reaction to liquid fuels. Available processes for the production of synthesis gas for GTL plants are based on steam reforming, partial oxidation or combinations hereof. Unfortunately, Fischer-Tropsch technology requires a great deal of energy and high temperatures ranging beyond 700° C.
Several years ago a research team at Texas A&M University conceived of a process for converting natural gas into hydrocarbon liquids using a “direct” conversion method. The process is considered “direct” because it does not require the generation of syngas. The process is essentially three reaction steps and two separation steps to produce hydrocarbon liquids. See Kenneth R. Hall, A New Gas to Liquids (GTL) or Gas to Ethylene (GTE) Technology, Catalysis Today, Vol. 106, pp. 243-46 (Oct. 15, 2005). However, the end result of this process is primarily ethylene and hydrogen, and not a liquid fuel.
To create a product that has greater commercial value, it is desirable to convert the gaseous alkanes into a liquid transportation fuel without need of a synthesis process or a metal catalyst. Further, a need exists for a process of quickly upgrading a naturally-occurring mixture of lower molecular weight fuels available at many production sites into a high octane transportation fuel without need of large chillers. Further, a need exists for a process for economically upgrading the lower molecular weight hydrocarbons from a gas stream, together into a high-octane liquid gasoline, or into a blend for transportation fuel, at or near a production site.